Knife gate valves are useful in a wide variety of fluid control applications, for example, in the petroleum and chemical industries, mining, power generation, as well as municipal and industrial water service utilities, wherever there is a need for a valve with a high flow capacity and relatively low head loss to control flows which need not be throttled.
Knife gate valves are generally understood as comprising a housing, which permits the valve to be positioned in a fluid conduit line, and a movable valve member in the form of a flat plate (the knife gate) that is slidably movable within the housing. The gate is movable transversely to the fluid flow direction between an open position, wherein the gate is removed from the fluid flow path through the housing to allow fluid flow through the valve, and a closed position, wherein the gate is inserted into the fluid flow path to block the fluid flow.
Resilient seated knife gate valves use pairs of seals mounted within the housing on opposite sides of and adjacent to the gate. The seals extend circumferentially around the fluid flow path. When the gate is in the closed position, the seals compressively engage its opposing surfaces and prevent leakage of fluid past the gate. When the knife gate is in the open position, the seals compressively engage one another around the fluid flow path and provide a radial seal which prevents leakage of fluid through the opening in the housing through which the gate moves.
Knife gate valves are normally operated in either a fully open or fully closed position. However, during valve opening and closing significant forces develop which tend to unseat and strip the seals from the housing. These forces include transient fluid dynamic forces which occur when the gate is partially open and the valve behaves as a venturi tube, causing accelerated fluid flow through the valve. Furthermore, the frictional forces between the seals and the gate generated when the gate moves between the seals impart significant shear forces to the seals tending to buckle them and strip them out of the housing and into the fluid flow path. The frictional forces arise largely due to the compressive engagement between the seals and the gate, which is required to ensure a fluid tight seal between them.
Seals for knife gates must endure significant compression, because they must be compressed against one another sufficiently to provide the radial seal preventing leakage when the valve is open, and then be compressed further to accommodate the knife gate when it is inserted between the seals to close the valve. The seals must endure linear compression on the order of 10% to effect the radial sealing of the valve, and further compression, up to approximately 30%, when compressively engaged by the gate.
The seals are advantageously formed of flexible, resilient material which is incompressible, i.e., if one portion of the seal is compressively deformed then another portion of the seal must be allowed to expand in reaction thereto. If the seals are not permitted room to expand, then they will not deform under the compressive loads of the gate and will transfer their compressive stress to the gate and the housing, preventing motion of the gate or damaging the housing or the seal.
In designing a seal for a knife gate valve, it is thus advantageous to develop a seal which is strong enough to resist unseating from the housing while being flexible and resilient so as to repeatedly deform as necessary to accommodate the motion of the gate and form an effective seal within the valve in both the open and closed configurations.